Hubble pictures from study of Comet Hyakutake released

ANN ARBOR -- University of Michigan astronomer Michael Combi has
released two images of the inner coma, or gassy head, of Comet
Hyakutake, which passed within about 16 million kilometers of Earth
in 1996. The computer-generated pictures were taken with NASA's
Hubble Space Telescope on April 4, 1996, during observations led by
Combi, a research scientist at the U-M College of Engineering.

These images, made using Hubble's Wide Field Planetary Camera 2,
were part of a study of water photochemistry in comets. Hydrogen
atoms are the most abundant gas in the entire coma of the comet.
They are produced when solar ultraviolet light divides molecules of
water, the major constituent of the nucleus of the comet. Using
Hubble's High Resolution Spectrograph, Combi and his colleagues
were able to determine that Hyakutake was churning out between 7
and 8 tons of water per second, by matching a computer model of the
comet the researchers had created earlier to the observations.

"The importance of such a detailed model is that it permits the
accurate calculation of the production rate of water from
observations," Combi said. The results appear in an article in the
Feb. 20, 1998, issue of the Astrophysical Journal (vol 494, pages
816-821).

The first image, shown in red, was taken through a narrow-band red
filter that shows only sunlight scattered by dust particles in the
inner coma of the comet. The second, shown in blue, was taken with
an ultraviolet "Woods" filter image that shows the distribution of
scattered ultraviolet radiation from hydrogen atoms in the inner
coma.

The inner yellow region near the center of the red dust image is
dominated by the contribution from the dust which shows sunward
directed spiral jets toward the upper right, and the thin straight
particle trail pointing toward the lower left. The trail was a
permanent feature of the comet around the time of its close
approach to the earth in late March and early April. Also barely
visible just beyond the lower left end of the trail are two of the
many condensations which were seen to travel slowly down the tail
are believed to be clumps of material released from the comet's
nucleus -- a 2-3 km chunk of dirty ice.

The inner white region of the blue image appears to show that the
hydrogen atoms like the dust might be preferentially ejected toward
the sunward or day side of the nucleus. However, this is not true.
The asymmetric ultraviolet radiation pattern is produced by a
roughly spherical distribution of hydrogen atoms because they are
so efficient at scattering the incoming solar ultraviolet light.
The atoms on the sunward side actually shadow the atoms on the
tailward or night side of the coma. The same detailed model
analysis of the coma which explains the expansion of the hydrogen
atoms in the coma also explains the appearance of the image.

Combi's team included Michael Brown of the California Institute of
Technology, Paul Feldman of the Johns Hopkins University, H. Uwe
Keller of the Max Planck Institute, Lindau, Robert Meier of the
Naval Research Laboratory, and William Smyth of Atmospheric and
Environmental Research, Inc.